The induction of cataracts is often an unfortunate and unavoidable consequence of conventional radiation therapy for head and neck or ocular tumors, whole-brain irradiation, and total-body irradiation prior to autologous bone marrow transplantation. Though not life-threatening, radiation-induced cataractogenesis represents a potentially serious sequelae of radiotherapy which can require surgical intervention. While the cellular and molecular mechanism(s) of radiation-induced cataractogenesis have not been clearly elucidated, damage to the genome at the time of exposure and subsequent proliferation of the radiosensitive cells in the germinative zone of the lens epithelium likely play a role in the process. Using a rat model, we have recently accumulated preliminary data which indicate that estrogen reduces the latent period and may increase the incidence and severity of radiation-induced cataracts. High estrogen levels are artificially induced in nonpregnant women using oral contraceptives, or in post-menopausal women on estrogen replacement therapy, and these groups may be at an increased risk for developing cataracts which are more severe or occur with a more rapid onset. Estrogens regulate several proteins involved in cell cycle control and apoptosis, and its metabolism results in the production of free radicals which may be genotoxic and mutagenic to mammalian cells. Thus, a novel hypothesis to be tested in the proposed studies is that estrogen alters cell cycle regulation, DNA double strand break induction or repair, and proliferation in irradiated lens cells. We shall also investigate the dose-time interactions of radiation and estradiol to better understand the mechanism of estrogen action, and we will determine whether estrogen-modulation of radiation cataractogenesis is estrogen receptor (ER)-mediated using knockout mice that are deficient in either ERalpha or ERbeta. The lens has frequently been used as a model for predicting delayed (late) effects in other irradiated tissues. Data obtained from the proposed study may demonstrate that the lens is a useful model for predicting late effects in other estrogen-responsive target tissues. Finally, the efficacy of utilizing a novel technique for small animal irradiations shall also be tested; in this study, using the Leksell Gamma Knife, only one eye shall be irradiated in each of the animals, with the contralateral eye serving as a control.